4 8 



STUDIES IN LUMINESCENCE. 



100 



and that no change that could be detected by our apparatus occurs in the 

 position of this maximum for 10 seconds after excitation has ceased. 1 



Fig. 43 also shows that if any change occurs in the form of the phospho- 

 rescence spectrum during decay, this change is extremely small. In other 



words, the different wave-lengths 

 of the phosphorescence spectrum 

 decay at the same rate. Table 12 

 will show r to what extent this con- 

 clusion is justified by the data. 

 Each column of this table refers to 

 the wave-length stated at the top, 

 and in it are tabulated the intensi- 

 ties for this wave-length at different 

 intervals after the end of excitation, 

 these intensities being expressed in 

 terms of the intensity at the end of 

 1.75 seconds as unity. If the phos- 

 phorescence spectrum remains ab- 

 solutely unaltered during decay, 

 all the numbers in this table that 

 lie in a given horizontal line should 

 be equal. 



The numbers in Table 12 corre- 

 sponding to 0.0 second are uncer- 

 tain ; first, because the observations 

 by which the initial phosphores- 

 cence was determined were them- 

 selves especially uncertain; and second, because the steepness of the curve 

 (Fig. 43) in the neighborhood of 0.48 /j. and 0.54 /x would cause a slight 

 change in the manner of plotting to produce a large change in the ordinates 

 at these wave-lengths. If the ratios tabulated for 0.0 second are left out 

 of consideration, the agreement between the remaining ratios, at 0.5 second 



.460 



.500 .540 



Fig- 43- 



.580// 



Phosphorescence spectrum of Sidot blende at 'lif- 

 erent times after removal of the exciting light. 



Table 12. 



Intensity of phosphorescence at different intervals after excitation 

 ceased, expressed in terms of the intensity at the end of 1.75 sec. 



had 



and 1 .0 second, is as close as could be expected. We conclude that, although 

 there is some indication of more rapid decay at the ultra edge of the phos- 

 phorescence spectrum, the probability is that all parts of the spectrum decay 

 at the same rate, and that the form of the phosphorescence spectrum 

 remains unchanged. 



'Studies of the phosphorescence spectrum of Sidot blende by a photographic method, which made it 

 possible to determine the position of the maximum at various times up to 90 seconds after excitation had 

 ceased and which confirm this conclusion and extend it, will be described in Chapter VIII. 



